Where does the music come from?

I have been asked, time and again, a question that I routinely ponder myself; Where does music come from? What comes first, the music or the lyrics? The simple answer is: It depends. Or, I don't know. I am always amazed that any group of musicians can agree on anything as personal and subjective as music enough to complete a song.
Of course the story would be different if it were one songwriter working alone, or at least making all the decisions her or his self, but when a band is required to agree to create a piece of music together and all the parts eventually come together and it works, it's amazing. It's mystifying. It's a wonderful thing to be a part of, and yet, after twenty something years, I'm still no closer to understanding how or why it works when it does. Or why sometimes everything clicks so effortlessly for one song, perhaps one that at first blush seemed like it would be so hard to write, but other times, and more often than not the songs that appear to be walks in the park end up being akin to trips to the dentist.
I suppose it's the mystery that makes the magic, and if we fully understood it, it might not make it so special after all.